1. Field of the Invention
The present invention relates generally to wireless digital communication. More specifically, the invention relates to a system and method for controlling the gain of a received communication signal in the digital domain.
2. Description of the Prior Art
A digital communication system typically transmits information or data using a continuous frequency carrier with modulation techniques that vary its amplitude, frequency or phase. The information to be transmitted is input in the form of a bit stream which is mapped onto a predetermined constellation that defines the modulation scheme. The mapping of the bit stream onto a plurality of symbols is referred to as modulation. Each symbol transmitted in a symbol duration represents a unique waveform. The symbol rate or simply the rate of the system is the rate at which symbols are transmitted over the communication channel.
Today, the most commonly used method for modulating data signals is quadrature amplitude modulation (QAM), which varies a predefined carrier frequency amplitude and phase according to an input signal. Other modulation techniques such as frequency modulation (FM), frequency shift keying (FSK), phase shift keying (PSK), binary phase shift keying (BPSK) contain little or no amplitude information when compared with the many types of QAM (64 QAM, 256 QAM, etc.) and quadrature phase shift keying(QPSK) which use the available bandwidth more efficiently by including amplitude information as part of the modulation.
QPSK and QAM techniques have information coded in both the phase and amplitude variations. In order to recover the amplitude modulated information accurately, the communication system receiver must have a linear response within the input signal range of the analog-to-digital converter (ADC) used to convert the received information, whether radio frequencies, intermediate frequencies or baseband frequencies, into a digital signal output for downstream digital signal processing. The dynamic range of the input signal at the antenna port may be very large. For example, in 3rd generation wireless protocols, the input signal dynamic range may exceed 70 dB.
A prior art technique for demodulating amplitude modulated signals is the use of a linear demodulator comprising an I and Q demodulator in conjunction with an automatic gain control (AGC) circuit to keep the input signal within the input range of the demodulator and/or within the input range of ADCs (analog to digital converters). An AGC circuit keeps an output within a linear operating region by adjusting the gain of an amplifier via feedback. Such a prior art AGC circuit 8 is shown in FIG. 1. The AGC comprises a voltage or current variable gain amplifier 10, a power computation processor 12 and a comparison circuit 14.
A signal input 16 to the AGC circuit 8 is coupled to the variable gain amplifier 10. The output power 18 is measured by the power computation processor 12 which produces an average or peak power measurement. The measured power is compared with a predefined value in the comparison circuit 14 which generates an error signal 20 corresponding to the difference in power level. The error signal 20 acts as negative feedback and controls the gain of the variable gain amplifier 10. In response to the error signal 20, the variable gain amplifier 10 controls the magnitude of the output signal 18 with reference to the input signal 16. The AGC circuit 8 maintains the output signal 18 within the linear operating region of the receiver and ADCs (not shown) employed to convert the analog signal to digital form.
While AGCs obviate input overloads, the individual components within the AGC circuit contribute their own distortions. The variable gain amplifier used in prior art AGC circuits is not ideal and suffers from a plurality of problems when reducing the amplifier design to a physical system. Problems such as amplifier dynamic range, linearity, noise figure vs. gain, input/output compression, constant phase vs. control signal, temperature stability, repeatability and others present a myriad of problems for a designer.
U.S. Pat. No. 5,533,064 discloses a digital radio receiver. An amplifier limiter receives an input and uses a plurality of limiting amplifiers to produce a constant amplitude signal. Using taps between the plurality of amplifiers, detectors are used to determine a log value of the input. A low pass filter removes harmonics from the constant amplitude signal. An orthogonal detector produces I and Q components of the constant amplitude signal. The I and Q components are converted to digital values. The log value is delayed and converted to a digital value. Using a log to linear converter the log value is converted into a digital value which is multiplied with the I and Q digital values respectively.
Impairments in the variable gain amplifier performance manifest themselves at the system level. Since the AGC circuit is usually a closed loop control system, any open loop gain variation in the design, such as nonlinearity, dynamic range, noise, etc., will reduce performance and cause instability downstream. Additionally, since an AGC circuit relies upon negative feedback, system speed is important, requiring a constant insertion phase.
Accordingly, there exists a need for a system and method that allows for precise AGC without the design limitations imposed by variable gain amplifiers and other components utilized in the prior art.